The present invention is generally directed to an implantable medical device, e.g., a cardiac stimulation device, and is particularly directed to an automatic capture/threshold pacing method for use in such a device.
Implantable cardiac stimulation devices are well known in the art. They include implantable pacemakers which provide stimulation pulses to cause a heart, which would normally beat too slowly or at an irregular rate, to beat at a controlled normal rate. They also include defibrillators which detect when the atria and/or the ventricles of the heart are in fibrillation or a pathologic rapid organized rhythm and apply cardioverting or defibrillating electrical energy to the heart to restore the heart to a normal rhythm. Implantable cardiac stimulation devices may also include the combined functions of a pacemaker and a defibrillator.
As is well known, implantable cardiac stimulation devices sense cardiac activity for monitoring the cardiac condition of the patient in which the device is implanted. By sensing the cardiac activity of the patient, the device is able to provide cardiac stimulation pulses when they are needed and inhibit the delivery of cardiac stimulation pulses at other times. This inhibition accomplishes two primary functions. Firstly, when the heart is intrinsically stimulated, its hemodynamics are generally improved. Secondly, inhibiting the delivery of a cardiac stimulation pulse reduces the battery current drain on that cycle and extends the life of the battery which powers and is located within the implantable cardiac stimulation device. Extending the battery life will therefore delay the need to explant and replace the cardiac stimulation device due to an expended battery. Generally, the circuitry used in implantable cardiac stimulation devices have been significantly improved since their introduction such that the major limitation of the battery life is primarily the number and amplitude of the pulses being delivered to a patient""s heart. Accordingly, it is preferable to minimize the number of pulses delivered by using this inhibition function and to minimize the amplitude of the pulses where this is clinically appropriate.
It is well known that the amplitude of a pulse that will reliably stimulate a patient""s heart, i.e., its threshold value, will change over time after implantation and will vary with the patient""s activity level and other physiological factors. To accommodate for these changes, pacemakers may be programmed manually by a medical practitioner to deliver a pulse at an amplitude well above an observed threshold value. To avoid wasting battery energy, the capability was developed to automatically adjust the pulse amplitude to accommodate for these long and short term physiological changes. In an existing device, the Affinity(copyright) DR, Model 5330 L/R Dual-Chamber Pulse Generator, manufactured by the assignee of the present invention, an AutoCapture(trademark) pacing system is provided. The User""s Manual, (copyright)1998 St. Jude Medical, which describes this capability is incorporated herein by reference. In this system, the threshold amplitude level is automatically determined for a predetermined duration level in a threshold search routine and capture is maintained by a capture verification routine. Once the threshold search routine has determined a pulse amplitude that will reliably stimulate, i.e., capture, the patient""s heart, the capture verification routine monitors signals from the patients heart to identify pulses that do not stimulate the patient""s heart (indicating a loss-of-capture). Should a loss-of-capture (LOC) occur, the capture verification routine will generate a large amplitude (e.g., 4.5 volt) backup pulse shortly after (typically within 80-100 milliseconds) the original (primary) stimulation pulse. This capture verification occurs on a pulse-by-pulse basis and thus, the patient""s heart will not miss a beat. However, while capture verification ensures the patient""s safety, the delivery of two stimulation pulses (with the second stimulation pulse typically being much larger in amplitude) is potentially wasteful of a limited resource, the battery capacity. To avoid this condition, the existing device, monitors for two consecutive loss-of-capture events and only increases the amplitude of the primary stimulation pulse should two consecutive loss-of-capture (LOC) events occur, i.e., according to a loss-of-capture criteria. This procedure is repeated, if necessary, until two consecutive pulses are captured, at which time a threshold search routine will occur. The threshold search routine decreases the primary pulse amplitude until capture is lost on two consecutive pulses and then, in a similar manner to that previously described, increases the pulse amplitude until two consecutive captures are detected. This is defined as the capture threshold. The primary pulse amplitude is then increased by a safety margin value, e.g., 0.3 volts, to ensure a primary pulse whose amplitude will exceed the threshold value and thus reliably capture the patient""s heart without the need for frequent backup pulses. In a copending, commonly-assigned U.S. patent application Ser. No. 09,483,908 to Paul A. Levine, entitled xe2x80x9cAn Implantable Cardiac Stimulation Device Having Autocapture/Autothreshold Capabilityxe2x80x9d, improved loss-of-capture criteria are disclosed which are based upon X out of the last Y beats, where Y is greater than 2 and X is less than Y. The Levine application is incorporated herein by reference in its entirety.
Whether a stimulation pulse successfully captures muscle, e.g., cardiac, tissue and thus causes the muscle to contract is related to an amplitude component, i.e., voltage or current, and a duration component of the stimulation pulse. This relationship was described in 1909 by Lapicque as a strength uration curve (see an exemplary curve 10 in FIG. 1) which is expressed by the equation:
I=IR*(1+dc/d)
where IR represents the current at the rheobase, i.e., the lowest current pulse (independent of duration) that can stimulate the body tissue and dc represents the chronaxie time duration, i.e., a duration at which stimulation requires twice the rheobase current value.
This relationship is readily apparent by setting d equal to dc which results in I=2*IR.
This equation can be adjusted to display voltage by multiplying each side by the lead impedance, resulting in:
V=VR*(1+dc/d)
The energy used for each pulse is a function of the amplitude level (i.e., voltage or current) and the duration of the delivered pulse as shown in the equation:
E=(V2*d)/R
where V is the amplitude of the voltage pulse, d is its duration and R is the lead impedance.
It has been observed and can be shown that the minimum energy point on the strength-duration curve is at a chronaxie point 12 (as shown in FIG. 1 which shows a prior art implementation of a stimulation energy curve), i.e., where the amplitude component is twice the rheobase 10 and the duration component is the chronaxie duration. Known automatic capture/threshold algorithms adjust the threshold amplitude, e.g., voltage, at a fixed duration, preferably the chronaxie duration. It appears that these algorithms are based on the assumption that changes in the strength-duration curve solely effect the rheobase, i.e., if the chronaxie is essentially fixed, the strength-duration curve will solely shift vertically during the life of the patient (see curve 14 relative to curve 10). Since the known existing automatic capture/threshold algorithms only alter the amplitude component (see stimulation energy curve 16), the belief that the chronaxie is xe2x80x9cfixedxe2x80x9d for a given patient is inherent in these algorithms. If in fact the chronaxie is fixed, an amplitude shift alone will result in the minimum energy dissipation since the stimulation point would shift from the chronaxie point 12 of strength-duration curve 10 to the chronaxie point 18 of the subsequent strength-duration curve 14.
Additionally, it is shown in a copending, commonly assigned PCT Patent application No. SE99/00813 to Nils Holmstrom entitled xe2x80x9cVariable Safety Margin in Autocapture Pacemakers,xe2x80x9d that due to the shape of the strength-duration curve, a larger safety margin is desirable with shorter duration stimulation pulses. Accordingly, the strength-duration curve (see FIG. 2) is divided into two regions having differently sized safety margins. The Holstrom application is incorporated herein by reference.
However, in contrast to the belief that the chronaxie was fixed, it has been noted by Raschack in an article entitled: xe2x80x9cDifferences in the cardiac actions of the calcium antagonists verapamil and nifedipinexe2x80x9d Arzneimittelforschung 1976;26 (7):1330-3, that the strength-duration curve xe2x80x9cis shifted to the right and the chronaxia (sic) value is significantly increased by verapamil.xe2x80x9d
The present inventor opines that such a horizontal shift, i.e., a chronaxie shift, or a combined horizontal and vertical shift, i.e., a shift in the rheobase and chronaxie, are not optimally accommodated by the prior art. Additionally, it is noted that since the energy dissipation is related to the square of the amplitude (voltage) of a stimulation pulse and only linearly related to its duration, amplitude-only increases to regain/maintain capture may be wasteful of battery capacity.
The altering of amplitude or duration have been examined in U.S. Pat. No. 5,697,956 to Bomzin, which is incorporated herein by reference. The Bomzin patent recognized that while the selection of stimulation energy levels was ideally related to the strength-duration curve for the patient""s cardiac tissue, optimal increases in energy levels should also take into account the battery voltage when voltage doublers (or triplers) are necessary to achieve a desired stimulation voltage. Accordingly, the Bornzin patent shows a stimulation energy curve (see FIG. 7 of Bornzin) that selectively increased either amplitude or duration (but not both) to increase the stimulation energy level while avoiding use of the voltage doublers (or triplers) when possible. However, the Bomzin patent does not show a system in which amplitude and duration were concurrently increased to increase stimulation energy.
U.S. Pat. No. 4,590,941 to Saulson et al. did show the use of stimulation pulses where the amplitude and the pulse width components of stimulation pulses were linearly related. However, Saulson did not show the use of these pulses in a system which included a method for automatic capture/threshold determination. In fact, this amplitude/duration relationship was not used in Saulson to improve capture of the patient""s heart. Specifically, Saulson disclosed a system in which its programmability was unidirectional and the only way to confirm the system""s programming was to monitor the stimulation pulse""s duration and thus infer the stimulation pulse""s amplitude due to this predefined relationship.
Therefore what is needed is a system that can adjust the amplitude and duration of stimulation pulses to improve immunity to shifts in the strength-duration curve and thus maintain capture in an automatic capture/threshold environment while minimizing battery depletion.
The present invention provides an improved system and method for performing automatic capture and threshold detection in an implantable cardiac stimulation device. The present invention defines a plurality of essentially linear stimulation energy curves that are selected as a function of two or more pulse duration regions. By selecting the energy curve dependent upon the pulse duration region, e.g., dependent upon the relationship of the present stimulation pulse to an amplitude-duration curve, increases in stimulation pulse energy can be selected that will have a decreased susceptibility to changes in the chronaxie and/or rheobase. Consequently, the ability to regain capture in the event of a loss-of-capture and the ability to maintain capture are improved.
A preferred implantable cardiac stimulation device is configured for stimulating a patient""s heart through at least one electrode implanted in electrical contact with selected cardiac tissue using a pulse generator configured for electrical coupling to the electrode and configured to generate stimulation pulses at a controlled energy level to thereby stimulate the patient""s heart, wherein the controlled energy level is defined by a set of characteristics including an amplitude component and a duration component. Additionally, a detection circuit is configured for electrical coupling to the electrode and configured to receive cardiac signals for determining the presence or absence of an evoked response to each of the stimulation pulses. A preferred device operates under control of a controller, coupled to the pulse generator, which increases the controlled energy level in response to a loss-of-capture criteria related to the absence of an evoked response. In such a case, the controlled energy level is increased from a first energy level (having a first amplitude component and a first duration component) to a second energy level (having a second amplitude component and a second duration component) where the change in amplitude and duration components is a function of at least two stimulation pulse duration regions determined by the controller.
In one preferred embodiment, three stimulation pulse duration regions are used which are determined by the controller according to first and second durations thresholds. In this embodiment, increases in stimulation energy (e.g., when a loss of capture criteria is met) are done according to the pulse duration region. In the first duration region less than the first duration threshold, the second amplitude component is set to be essentially the same as the first amplitude component and the second duration component is set to exceed the first duration component. In the second duration region between the first duration threshold and the second duration threshold, the second amplitude component is set to exceed the first amplitude component and the second duration component is set to exceed the first duration component. In the third duration region greater than the second duration threshold, the second amplitude component is set to exceed the first amplitude component and the second duration component is set to be essentially the same as the first duration component.
In a further aspect of the present invention, the chronaxie and rheobase of the strength-duration curve are periodically determined and the pulse duration regions are determined accordingly.
The novel features of the invention are set forth with particularity in the appended claims. The invention will be best understood from the following description when read in conjunction with the accompanying drawings.